A distributor-type electronic control fuel-injection pump is known as one type of fuel-injection pumps for use in a diesel engine. Japanese laid-open utility model publication No. 123273/1986 discloses a distributor-type fuel-injection pump wherein a distributor head includes a spill passage to provide a communication between a high pressure chamber, the volume of which is variable in response to reciprocating motion of a plunger, and a low pressure chamber (cam chamber). A solenoid valve is incorporated into the spill passage and is opened and closed to control the timing of initiating and stopping fuel delivery and thus, the amount and timing of fuel to be injected.
In such a prior art pump, a cam is used to reciprocate the plunger, and a change in the speed of the cam results in a corresponding change in the timing of fuel injection and thus, the amount of fuel injected. To this end, the cam is rotated at a constant speed so as to controllably initiate and stop fuel injection. However, if the cam is constantly rotated at a high speed, then engine output increases when an engine is ooperated at high speed and high load, but noise increases during low speed and low load operation. On the other hand, if the cam is rotated at a low speed, then the sound of knock decreases during low speed and low load operation, but the engine output decreases during high speed and high load operation.
To overcome this, Japanese laid-open patent publication No. 201330/1988 proposes a distributor-type electronic control fuel-injection pump wherein a cam is used to reciprocatingly move a plunger and configured to provide a high-speed portion by which the speed of the cam is kept constant during high speed operation, and a low-speed portion by which the speed of the cam is kept constant during low speed operation. This controls the amount of fuel to be injected.
When a normally open solenoid valve is incorporated into the spill passage to control the amount of fuel to be injected, the solenoid is energized to lift the valve so as to close the spill port during a discharge cycle where the plunger is lifted. This results in an increase in the pressure of fuel in the high pressure chamber, and fuel delivery is initiated. When the solenoid is deenergized, the valve is operable to open the spill port. This allows the fuel to flow from the high pressure chamber to the lower pressure chamber so as to stop fuel delivery.
During such an operation, the solenoid is operable in a rapid fashion since it is electrically actuated. On the other hand, those valves (for example, a poppet valve and a needle valve) that are actuated by an armature is mechanically slidably moved and therefore, relatively slowly operated. A delay in the response is concerned with the amount of fuel injected. Valve lift characteristics vary with the amount of fuel injected as shown in FIG. 4A. Such characteristics need not be taken for consideration with a normal injection system used in the prior art. With a high speed injection system or a high speed-direct injection system, it is inevitable, however, that a constant amount of fuel can not be injected at a high idle zone or at high speed/low load zone including governing zone (for example, when a vehicle is travelling on a down hill after passing through the top of the hill), as shown by shade lines in FIG. 5. A high idle operating zone is between 4000 and 4600 rpm with direct injection and between 5000 and 5400 rpm with indirect injection.
This is due to the fact that fuel is injected by a throttle action before the passage has completely been closed (pre-flow effect) or fuel continues to be injected while the valve is being opened (after-flow effect) even if the period during which the solenoid valve is completely closed (the valve is fully lifted) is substantially zero in response to an increase in the speed of rotation of the pump. It is clear that such a pre-flow effect or an after-flow effect often occurs when an engine is rotated at a higher speed, when a cam is operable to supply a greater amount of fuel, and when fuel is injected directly into engine cylinders. Under such a condition, high idle or high speed operation can not be carried out if a zone in which a small amount of fuel is injected is overlapped with a zone in which a constant amount of fuel can not be injected. This presents a serious problem in that ninety persent of vehicle operation is an idle operation.
In order to inject a small amount of fuel at a high idle zone, the solenoid is energized in a manner to open the valve before it is fully lifted as shown in FIG. 4B, taking into account a delay in the response of the valve. In such a way, the valve, say, "floats", and fuel is injected in a rather inconsistent manner. This adversely affects a cycle of fuel injection. Therefore, it is not practical.
It is an object of the present invention to overcome the foregoing problems and to provide a method for controlling the flow rate of fuel in a distributor-type electronic control fuel-injection pump, which is capable of constantly delivering a small amount of fuel during high speed operation or at high idle operating or other zones, and which is effective for a high speed-direct injection system.